1. Field of the Invention
The present invention relates, in general, to optical scanning and, more particularly, to the control of pixel clock timing in optical scanning devices for maintaining uniform pixel spacing across a scan line.
2. Description of Related Art
Optical scanning devices are found in many types of office machines, such as facsimile machines, laser printers, optical scanners, etc. A typical optical scanning device comprises a light source producing a light beam and a rotating, reflective surface deflecting the light beam to scan across a desired surface. The light beam scans the surface one line at a time to read or write a composite image. In the image reading mode, the light beam is scanned across an image surface and the intensity of light reflected from the image is read and recorded to represent the image. In the printing mode, the light beam, encoded with image information, is scanned across a photoreceptor surface. The light beam is focused to a spot which moves across the photoreceptor surface as the light beam is turned on and off in accordance with the image information to define pixels of the image.
The light beam is typically deflected at a constant angular velocity by the rotating, reflective surface. Since the scanned surface is planar, not all portions of the scanned surface are equidistant from the reflective surface. Therefore, the spot made by the light beam moves across the scanned (flat) surface at a changing linear speed during a scan cycle, the spot-speed being higher at the beginning and end of the scan where regions of the scanned surface are farther from the reflective surface than more central regions located at the middle of the scan. This means that when data representing an image is supplied at a constant timing rate to the light source, the spacing of the pixels along the scan line will be greater at the higher spot-speed regions (the extremities of the scan) than at the middle portions of the scan. The non-uniformity in pixel spacing arising from variation in spot-speed is undesirable for most printing applications. This is because non-uniformity in pixel spacing across the scan line, results in a distorted raster which, in turn, results in dimensional distortion of the image.
One can compensate for variations in the spot-speed either optically or electronically. Optical compensation typically involves the incorporation of a so-called F-theta lens interposed along the light beam between the reflective surface and the scanned surface. The F-theta lens, however, introduces aberrations which cause field curvature in the image plane, thereby adversely affecting the focusing of the spot.
Electronic compensation involves the adjustment of the timing rate at which image data pulses are supplied to the light source. This typically requires adjusting the control voltage of a voltage controlled oscillator ("VCO") responsible for generating the pixel clock signal used to gate the image data pulses and turn the light beam on and off at each pixel location along the scan line. Varying the clock frequency, and thereby the timing of individual data pulses, serves to control pixel placement along the scan line.
Heretofore, the means to generate the VCO control voltage have been complex and expensive for acceptable pixel spacing uniformity.